docs: def prepare_untran(feat_type, tgt_dir, untran_dir):
      """ Preprocesses untranscribed audio."""
      org_dir = str(untran_dir)
      wav_dir = os.path.join(str(tgt_dir), "wav", "untranscribed")
      feat_dir = os.path.join(str(tgt_dir), "feat", "untranscribed")
      if not os.path.isdir(wav_dir):
          os.makedirs(wav_dir)
      if not os.path.isdir(feat_dir):
          os.makedirs(feat_dir)
  
      # Standardize into wav files
      for fn in os.listdir(org_dir):
          in_path = os.path.join(org_dir, fn)
          prefix, _ = os.path.splitext(fn)
          mono16k_wav_path = os.path.join(wav_dir, "%s.wav" % prefix)
          if not os.path.isfile(mono16k_wav_path):
              feat_extract.convert_wav(Path(in_path), Path(mono16k_wav_path))
  
      # Split up the wavs and write prefixes to prefix file.
      wav_fns = os.listdir(wav_dir)
      with (tgt_dir / "untranscribed_prefixes.txt").open("w") as prefix_f:
          for fn in wav_fns:
              in_fn = os.path.join(wav_dir, fn)
              prefix, _ = os.path.splitext(fn)
              # Split into sub-wavs and perform feat extraction.
              split_id = 0
              start, end = 0, 10 #in seconds
              length = utils.wav_length(in_fn)
              while True:
                  sub_wav_prefix = "{}.{}".format(prefix, split_id)
                  print(sub_wav_prefix, file=prefix_f)
                  out_fn = os.path.join(feat_dir, "{}.wav".format(sub_wav_prefix))
                  start_time = start * ureg.seconds
                  end_time = end * ureg.seconds
                  if not Path(out_fn).is_file():
                      wav.trim_wav_ms(Path(in_fn), Path(out_fn),
                                      start_time.to(ureg.milliseconds).magnitude,
                                      end_time.to(ureg.milliseconds).magnitude)
                  if end > length:
                      break
                  start += 10
                  end += 10
                  split_id += 1
  
      # Do feat extraction.
      feat_extract.from_dir(Path(os.path.join(feat_dir)), feat_type=feat_type)